Pulse plating process for deposition of gold-tin alloy

ABSTRACT

The invention relates to a solution for use in connection with the deposition of a gold-tin alloy on an electroplatable substrate. This solution generally includes water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, and complexed gold ions. Optionally, a deposit conditioning agent can be added to the solution to assist in providing a uniform and bright deposit. The solution has a pH of between about 2 and about 10 and the deposit has a gold content of between about 65% to about 90% by weight and a tin content between about 10% and about 35% by weight. An advantageous way for providing a stable gold-tin deposit is by a pulse plating technique.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 10/847,468 filed May 18, 2004 which claims the benefit of U.S. provisional application 60/472,826 filed May 21, 2003, and a continuation-in-part of U.S. application Ser. No. 11/337,246 filed Jan. 19, 2006 which claims the benefit of provisional application 60/645,949 filed Jan. 21, 2005. The entire content of each prior application is expressly incorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

Alloys of gold and tin, particularly in the compositional range of about 75 to 80 percent gold by weight, are useful as solders for the interconnection of electronic components. Gold-tin alloys are also useful in many microelectronic applications including chip bonding and wafer bump plating. The 80-20 wt % (70-30 at %) gold-tin eutectic alloy is particularly desirable as a solder. The alloy may be applied by vacuum deposition or as a solid preform, however, electrodeposition, due to its low cost, is a preferred method of application. It is generally acknowledged that the ability to electroplate such materials allows the flexibility to deposit overall or in selected areas at will, and to adjust deposit thicknesses as required. For these reasons, much effort has been made to develop workable electroplating systems for these alloys, and numerous references are provided in the literature about such systems.

Prior art electroplating baths for the deposition of gold-tin alloy have been found to be incapable of depositing the eutectic alloy over a usable current density range. This was clearly demonstrated in “Film growth characterization of pulse electro deposited Au/Sn tin films” by Djurfors and Ivey (GaAs MANTECH, 2001), where in FIG. 1 they show a step transition from 16 at % Sn to 50 at % at a current density of around 1.5 mA/cm². According to the authors this is a result of the deposition of two distinct phases: Au₅Sn (16 at % Sn) at low current density and AuSn (50 at % Sn) at high current density. This has been further confirmed by test work which has shown that prior art electrolytes will not typically yield the desired eutectic alloy.

The prior art electrolytes, using complexing agents such as citric acid, pyrophosphate, gluconic acid, ethylene diamine tetra acetic acid (“EDTA”), and the like, typically yield alloys which are either tin rich (<50% Au) or gold rich (95% Au), or have tin rich or gold rich regions at different current densities. An 80/20 wt % eutectic gold-tin alloy cannot be deposited over a usable current density range. Moreover, many prior art baths suffer from poor stability making them of little practical interest.

U.S. Pat. No. 4,634,505 by Kuhn, et al. describes a bath using trivalent cyanide gold complex and a tin IV oxalate complex, which operates at pH below 3, but this bath gives deposits with less than 1% Sn, and is not useful for depositing a eutectic alloy.

U.S. Pat. No. 4,013,523 by Stevens et al. describes a bath using trivalent gold complex and tin as stannic halide complex. The pH is less than 3 and the bath allegedly is capable of depositing an 80-20 wt % gold alloy.

U.S. Patent Application No. 2002063063-A1 by Uchida et al. describes a non-cyanide formulation where the gold complexes used include gold chloride, gold sulfite, gold thiosulfite among others. The electrolyte includes stannic and stannous salts of sulfonic acids, sulfosuccinates, chlorides, sulfates, oxides and oxalates. The tin is complexed with EDTA, DTPA, NTA, IDA, IDP, HEDTA, citric acid, tartaric acid, gluconic acid, and glucoheptonic acid among others. The deposit is brightened by a cationic macromolecular surfactant.

Japanese patent application 56136994 describes a solution which uses sulfite gold complex in combination with stannous tin pyrophosphate complex at a pH of 7 to 13.

German patent DE 4406434 describes a solution using the trivalent cyanide gold complex in conjunction with stannic tin complexes. The pH is 3-14 and an 80-20 eutectic alloy deposit allegedly may be provided.

U.S. Pat. No. 6,245,208 by Ivey et al. discloses a non-cyanide formulation which uses gold chloride in combination with sodium sulfite, stannous tin, a complexing agent (ammonium citrate), and uses ascorbic acid as a stabilizer. Eutectic alloy deposits are claimed and bath stability on the order of weeks is reported.

As noted above these baths are not always stable and have been found to be insufficient in providing eutectic gold tin alloys on electroplatable substrates.

Djurfors and Ivey (GaAs MANTECH, 2001) describe a method in which on-off pulsing current is used and alternating plating period of higher and lower on-off pulsing current are applied during electroplating. The said time periods are on the order of minutes and a deposit consisting of distinct layers of different alloy composition gold tin alloy are formed. This layered deposit may then be annealed to form a homogeneous deposit of the desire composition.

Accordingly, there is a need for a process for electroplating deposits of a eutectic gold-tin alloy on various substrates, and this is now provided by the present invention.

SUMMARY OF THE INVENTION

The invention relates to a method for electroplating gold-tin alloy deposits on a substrate where the gold content is between about 65% to less than 90% by weight and a tin content of above 10% to about 35% by weight. The method comprises contacting the substrate with one of the solutions described herein, such solutions generally comprising water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, and complexed gold ions, with the solution having a pH of between about 2 and about 10 so that the deposit will have the gold and tin contents mentioned above. Preferably, the alloy content is near the eutectic compositions of 80% by weight gold and 20% by weight tin, but can be in the range of 75% to less than 85% gold and 15% to 25% tin.

The gold-tin deposits are generally matte and of the desired alloy content. If desired, the solution can further comprise a deposit conditioning agent in an amount sufficient to enhance the appearance or uniformity of the gold-tin deposit. Preferred deposit conditioning agents include 2,2′-dipyridyl, polyethylene imines, and many others and are present in an amount effective to enhance the smoothness or brightness of the gold-tin deposit.

A pulsed current is applied though the solution to provide the gold-tin alloy electrodeposit upon the substrate. This is used to achieve stable deposits of the desired gold-tin alloy. The pulsed current preferably comprises an uninterrupted, sequential, off-on, continuously repeating pulsing sequence over a base current applied across plating cell electrodes that applies high and low current densities in the solution for predetermined millisecond time periods. The pulsed current is generally off from about 1 to 25 milliseconds and then is turned on for about 1 to 25 milliseconds to provide the pulsed current. A typical base current density is between about 1 ASF and about 20 ASF with the pulsed current density ranging from 0.1 to 8 ASF. A preferred base current density is between about 2 ASF and about 10 ASF with the pulsed current density ranging from 0.2 to 5 ASF. The pulsed current is preferably on for a shorter time than when it is off. For example, the pulsed current can be off for between about 5 milliseconds and about 10 milliseconds followed by being on for about 1 millisecond to about 4 milliseconds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has now been found that an alloy with a significant tin content, exemplified by the eutectic 80/20 wt % gold-tin alloy, can be deposited. Thus, while alloys such as about 70% gold—about 30% tin and about 90% gold—about 10% tin are obtainable, the eutectic alloy, or as close to the eutectic alloy as possible, is preferred due to the well known advantages of such an alloy.

As used herein, the term “about,” when modifying a numerical value, is used to refer to a variance ranging from 0% to 20% of the value of the number being modified. For example, the term “about 20” refers to a numerical value of 20 (0% variance) or a numerical range of 18-22 (10% variance) or a maximal numerical range of 16 to 24 (20% variance). As will be clear to others skilled in the art, other numerical ranges are contemplated by the invention.

In one embodiment, the electrolyte solution comprises water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, and complexed gold ions. If desired, a deposit conditioning agent can be added to the solution in an amount sufficient to enhance the appearance or uniformity of the gold-tin deposit. As noted above, any one of a wide variety of such agents can be used, including 2,2′-dipyridyl or polyethylene imines, and are present in an amount effective to enhance the smoothness or brightness of the gold-tin deposit. The skilled artisan can conduct routine tests to determine the optimum agents for any particular electrolyte.

The tin ions can be added in any solution soluble form that provides stannous ions. Any two-valent tin salt, including sulfate, chloride, methane sulfonate, oxalate, or any other suitable stannous tin salt, can be used to provide these stannous ions, and the specific tin salt is not critical. Stannic tin may also be added to the solution; however, some stannous tin must be present in the electrolyte for the invention to function properly. This is because a non-alkaline electrolyte containing only stannic tin ions will not provide any appreciable amounts of tin in the deposit and will instead result in a deposit that is almost pure gold. The stannous tin ion concentration in the inventive solution is between about 1 g/l and about 20 g/l and more preferably between about 2 g/l and about 10 g/l.

Also, the concentration of stannous ions may be adjusted in relation to the gold ion concentration to provide the desired alloy. One of ordinary skill in the art can optimize the metal concentrations in any particular solution to obtain the desired gold-tin alloy.

An antioxidant or reducing agent is preferably included to help maintain the tin ions as stannous tin. Catechol, hydroquinone, or phenolsulfonic acid, or other agents known it the art to prevent tin oxidation can be used with catechol being preferred. The amount of this agent is between about 0.1 g/l and about 5 g/l, and preferably between about 0.5 g/l and 2 g/l.

A complexing agent is present in the solution to assist in rendering and maintaining the stannous tin ions soluble in the solution at the operational pH. Any suitable organic acid can be used for this purpose. Examples of complexing agents useful in the present invention include but are not limited to oxalic acid, citric acid, ascorbic acid, gluconic acid, malonic acid, and iminodiacetic acid. Generally, carboxylic acids are preferred, but iminodiacetic acid and ascorbic acid, which are not carboxylic acids, are also preferred complexers. Moreover, any other complexing agent that can complex the stannous tin in the solution can be used. Stannous ion is known to be complexed by various organic ligands. In the electrolytes of this invention, citrates, oxalates, and iminodiacetates were found to be suitable.

The complexing agent is present in the solution in at least a sufficient concentration to maintain the stannous tin soluble at the solution pH. Additionally, it is desirable to maintain an excess of complexing agent beyond the minimum concentration to improve solution conductivity and to provide pH buffering. The complexer concentration is typically between about 10 g/l and about 300 g/l and is most preferably between about 40 g/l and about 150 g/l.

The gold ions are preferably provided in the solution as a gold cyanide complex, most preferably monovalent gold cyanide, although trivalent gold cyanide may also be used. Non-cyanide sulfite gold complex can also be used in these solution when short solution lives are acceptable; otherwise this complex would not be preferred as the stability of this complex is inferior to the others. The most preferred gold ion complex is potassium gold cyanide and the preferred concentration of the gold ion complex is between about 2 g/l and about 20 g/l and most preferably between about 3 g/l and about 10 g/l.

The gold-tin deposits are generally matte and of the desired alloy content. For enhanced deposit characteristics, the solution can optionally include a deposit conditioning agent. Such agents include 2,2′-dipyridyl (WO2005/118917), polyethylene imines, peptone (U.S. Pat. No. 6,245,208), or piperazine, piperidine and morpholine in combination with arsenic oxide or arsenic compounds (German patent publication DE 4406434). Also useful is a cationic macromolecular surfactant known as SHALLOL DC902P (trade name, made by DAI-ICHI KOGYO SEIYAKU Co., Ltd.—see U.S. Pat. No. 6,544,398). SHALLOL DC902P contains as an active ingredient the quaternary ammonium salt of a diallylamine polymer. Other suitable agents include 0.5 ml/l of TRITON QS-15 (U.S. Pat. No. 4,013,523), or even 0.5 g/l saccharin. These compounds are present in an amount effective to enhance the smoothness or brightness of the gold-tin deposit. Preferred deposit conditioning agents include 2,2′-dipyridyl in an amount between about 0.1 and 1 g/l and polyethylene imines in an amount between about 0.2 and 10 ml/l. The skilled artisan can readily determine the appropriate amount of such agent or agents to use for any particular formulation by routine testing.

Other additives can be added to the solution to modify the grain structure of the deposit. These include metallic additives such as nickel, cobalt, arsenic, lead, thallium, or selenium. Organic additives such as those described in U.S. Patent Application No. 2002063063 may also be used, if desired. Additionally, other salts or buffers may be optionally added to the electrolyte to improve conductivity or pH stability. Examples, of such additives include simple salts such as potassium MSA, potassium sulfate, as well as others well known in the art.

The pH of the electrolyte is between about 2 and about 10, is generally less than 8 and is most preferably between about 3 and about 5.5. The preferred pH of the solution is generally dependent upon the gold complex that is used. For instance, potassium gold cyanide is not stable below a pH of 3.5, but a trivalent cyanide gold complex is stable at lower pH values. Sulfite gold complexes are generally not stable below pH 6 and are most stable at pH 8 and higher. Since the solution of the present invention is useful in microelectronics applications, it is desirable to have a pH of less than 8 and preferably less than 7 to prevent solution attack on photoresist masks that are often applied to the electrodeposition substrates. Additionally, it has been found that deposit appearance of tin containing alloys begins to degrade at pH values greater than about 4.7. The pH can be adjusted to the desired ranges using an acid or base, as necessary.

In the most preferred solutions, gold is contained in the form of a soluble cyanide complex, and tin in the form of a complex of stannous ion with a suitable organic ligand.

The solution temperature is typically between about 20° C. and about 70° C. and is most preferably between about 38° C. and about 50° C. Typically, temperature has a direct effect on the composition of the deposited alloy, with higher temperature resulting in higher gold concentrations.

The electrolyte of the present invention may be operated using insoluble anodes including platinized titanium, platinized niobium, or iridium oxide electrode. It is also possible to use soluble anodes, however, this is not typically practiced in precious metals plating.

The desired deposits are provided by a pulse plating technique. Gold/Tin deposits can be controlled to some extent by bath chemistry, but plating at varying current densities can change the proportions of the metals in the deposit. For example, at a given gold to tin ratio in the bath, the alloy of the plated deposit will vary greatly by increasing or decreasing the current density. At higher current densities, the deposit tends to be higher in gold tin content, while at lower current densities, the deposit tends to be higher in gold tin. Thus, variations from the desired eutectic 80 gold-20 tin deposit can be encountered depending upon applied current density. Furthermore, it is not always possible to plate at optimum current densities to obtain the desired alloy. i.e., burning can occur under high current density conditions, while dullness of the deposit is obtained under low current density conditions. Gold to tin ratios, pH, additives and temperature of the solution also have an effect on the plated alloy, and are controllable to a large degree, but often a more precise control is desired.

This additional control of alloy deposit is now provided by the present invention. By applying a pulse plating technique, several advantages are obtained. The content of the alloy deposit can be more accurately controlled by varying the applied current density.

The process is primarily suited for plating typical substrates such as metallized silicon wafers with photo resist masking or a metallic substrate that has a photo resist masking. The metallic or metallized portions are readily platable while the masking is not platable, thus enabling the deposited metal to be applied in a circuit or other metal pattern.

As noted, the pulsed current preferably comprises an uninterrupted, sequential, off-on, continuously repeating pulsing sequence over a base current applied across plating cell electrodes that applies high and low current densities in the solution. One way to do this is to apply a pulsed current that is at a low value from about 1 to 25 milliseconds and then is increased to a higher current density for a second period of about 1 to 25 milliseconds to provide the pulsed current. A typical base current is between about 1 ASF and about 20 ASF with the pulsed current ranging from 0.1 to 8 ASF. A preferred base current is between about 2 ASF and about 10 ASF with the pulsed current ranging from about 0.2 ASF to about 5 ASF. The pulsed current is preferably on for a shorter time than when it is off.

The most preferred embodiment includes pulse plating using a combination of a constant current with a superimposed pulsed current. The use of a constant D.C. current density at low current densities of about 4 ASF to about 5 ASF produces a gold rich alloy at good deposition rate. By adding a pulsed current, such as about 1 ASF to about 1.5 ASF at about 2 seconds on and about 8 seconds off using a separate rectifier wired in parallel, the electrolyte is refreshed at the plated interface and then is spiked at the higher current density, thus reducing the possibility of burning the deposit. An additional benefit is the ability to control the tin accurately in the deposit by simply increasing the current on the pulse rectifier. For example, the pulsed current can be off for between about 5 milliseconds and about 10 milliseconds followed by being on for 1 to 4 milliseconds to achieve optimum results for the bath chemistries disclosed herein.

With the present invention, the interval of the direct current pulses is on the order of milliseconds to seconds. The alloy composition may be adjusted by varying the relative lengths of the intervals. The desired pulsing direct current may be obtain by connecting a constant current DC rectifier in parallel with a pulsing (on/off) constant current power supply. This configuration produces a current source which fluctuates between high and low current densities. By way of non-limiting example, values which have been successfully used are a low current density of 5 ASF for 8 milliseconds and a high current density of 6.25 ASF for 2 milliseconds. Other values can be used and the previous conditions are given as one preferred example only. A skilled artisan can determine by routine testing the most preferred pulse plating conditions for any particular electroplating solution to achieve the desired alloy content of the deposit.

Other electroplating processes of the invention may include two power supplies connected in series to produce two alternating voltages, or a programmable power supply such as a galvanostat or potentiostat used to supply the current for the plating process. Moreover, other wave forms, such as a sinusoidal wave form, can be superimposed on a direct current and act in the same fashion as the pulsed current processing technique described herein.

EXAMPLES

The following examples illustrate useful embodiments of the invention.

Example 1

A eutectic gold-tin alloy electrodeposit is obtained from the following solution and under the following electroplating conditions. Tin (as tin sulfate) 6.4 g/l Gold (as potassium gold cyanide)   8 g/l Polyethylene Imine (1200 MW) 4 ml/l 10% solution Buffer  70 g/l Catechol   1 g/l pH adjusted with KOH 3.8

This electrolyte solution was utilized to demonstrate the use of pulses of direct current of different current densities to electrodeposit gold-tin alloys. Briefly, copper coupons (1.25″×1.25″) were plated in the electrolyte at a temperature of about 110° F. (45° C.).

Coupon #1 was plated using a conventional 5 ASF DC current and yielded a deposit of 86 wt % gold as measured by EDAX; the melting temperature was measured as 590° F. (310° C.) to 610° F. (320° C.).

Coupon #2 was plated alternately at 5 ASF for 8 ms and 2 ms at 6.25 ASF. This yielded a deposit of 76 wt % gold as measured by EDAX; the melting temperature was measured as about 535° F. (280° C.).

The results demonstrate that the pulsed plating technique of the present invention can help provide a more uniform alloy deposit, one that is closer to the eutectic composition so that a lower melting (or reflow) temperature can be used.

Example 2

This experiment was designed to test the effect of pulse rectification modification on increasing tin in the deposit. As a first step, the following one liter solution was prepared with a Gold:Tin ratio of 1:1.25. Tripotassium citrate 75 g/l Citric acid 105 g/l Au 8 g/l Tin 6.4 g/l pH (adjusted with KOH) 3.8

0.16 grams of 2,2′-dipyridyl was included as a deposit conditioning agent

The bath was maintained at a temperature of 110° F. (43.33° C.) and agitated with a stir bar during electroplating. A 1.25″×1.25″ copper lid was utilized for the electroplating of all test samples. The results are as follows:

Sample 1A was electroplated at 5 ASF (No Pulse). The resultant gold:tin alloy had a thickness of 5 microns, a composition of 82.1% gold, an SEM-measured gold value of 86.2% and a melting temperature of 590-608° F. (310-320° C.).

Sample 1B was electroplated at 5 ASF with pulsing at 2.5 ASF on for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 5 microns, a composition of 72.7% gold, an SEM-measured gold value of 80.06% and a melting temperature of 572° F. (300° C.).

Sample 1C was electroplated at 5 ASF with pulsing at 5 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 5 microns, a composition of 70.6% gold, an SEM-measured gold value of 83.48% and did not remelt to liquid.

Sample 1D was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 5 microns, a composition of 75.9% gold, an SEM-measured gold value of 83.24% and a melting temperature of 617° F. (325° F.).

Sample 1E was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 10 microns, a composition of 75.5% gold, an SEM-measured gold value of 75.94% and a melting temperature of 527° C. (275° F.).

Sample 1F was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 15 microns, a composition of 76.7% gold, an SEM-measured gold value of 74.96% and a melting temperature of 572° F. (300° F.).

Sample 1G was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 20 microns, a composition of 77.2% gold, an SEM-measured gold value of 76.1 and a melting temperature of 536° F. (280° C.).

Sample 1H was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 25 microns, a composition of 77.4% gold. an SEM-measured gold value of 73.8% and a melting temperature of 527° F. (275° F.).

Alloy thickness and measurement of percent Au was determined by X-ray diffraction and represent an average from 5 measurements. (Numerical values with sample C were obtained with 3 measurements) Scanning Electron Microscopy with Energy Dispersive X-ray analysis was also used to separately measure the gold percentage of the alloy.

These results of the experiment also indicate that the pulsed plating technique of the present invention can help provide a more uniform alloy deposit, one that is closer to the eutectic composition so that a lower melting (or reflow) temperature can be used.

As noted, the deposits usually have a matte appearance, but this can be improved by conducting a reflow operation to provide a uniform bright deposit. An alternative way to improve appearance is to include in the electrolyte one of the deposit conditioning agents described herein. In either situation, an alloy deposit that is close to the gold-tin eutectic composition can be obtained by the present invention. 

1. A method for electroplating of a gold-tin alloy deposit on a substrate which comprises contacting the substrate with a solution comprising water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, and complexed gold ions, with the solution having a pH of between about 2 and about 10, and applying a pulsed current though the solution to provide a gold-tin alloy electrodeposit upon the substrate, wherein the pulsed current comprises an uninterrupted, sequential, off-on, continuously repeating pulsing sequence upon a base current applied across plating cell electrodes that applies high and low current densities in the solution for predetermined millisecond time periods.
 2. The method of claim 1, wherein the complexed gold ions are present in the form of a solution soluble cyanide complex, and the stannous tin ions are present is in the form of a solution soluble organotin complex.
 3. The method of claim 2, wherein the gold complex is monovalent gold cyanide or trivalent gold cyanide, and the organotin complex is stannous citrate, stannous oxalate, or stannous iminodiacetate.
 4. The method of claim 1, wherein the deposit has a gold content of between about 65% and about 90 % by weight and a tin content of between about 10% and about 35% by weight.
 5. The method of claim 1, wherein the complexed gold ions are gold cyanide or gold sulfite complexes and are present in an amount of between about 2 g/l and about 20 g/l and the solution has a pH of about 8 or less.
 6. The method of claim 1, which further comprises a deposit conditioning agent in an amount sufficient to enhance the appearance or uniformity of the gold-tin deposit.
 7. The method of claim 6 wherein the deposit conditioning agent is 2,2′-dipyridyl, an polyethylene imines, (others) and are present in an amount effective to enhance the smoothness or brightness of the gold-tin deposit.
 8. The method of claim I which further comprises an antioxidant in an amount sufficient to maintain the tin ions as stannous tin ions, wherein the antioxidant is catechol, hydroquinone, or phenolsulfonic acid and is present in the solution in an amount of between about 0.1 g/l and about 20 g/l.
 9. The method of claim 1, further comprising one or more of thallous, plumbous or arsenious ions in an amount effective to provide grain refinement to the deposit.
 10. The method of claim 1, wherein the pulsed current is off from about 1 to about 25 milliseconds and then is on for about 1 millisecond to about 25 milliseconds to provide the pulsed current.
 11. The method of claim 1, wherein the base current is between about 1 ASF and about 20 ASF and the pulsed current ranges from about 0.1 ASF to about 8 ASF.
 12. The method of claim 1 1, wherein the base current is between about 2 ASF and about 10 ASF and the pulsed current ranges from about 0.2 ASF to about 5 ASF.
 13. The method of claim 11, wherein the pulsed current is on for a shorter time than when it is off.
 14. The method of claim 13, wherein the additional current is off for between about 5 milliseconds and about 10 milliseconds followed by being on for about 1 millisecond to about 4 milliseconds. 